Method for producing acid chlorides

ABSTRACT

The invention relates to a method for producing acid chlorides by converting carboxylic acids with carbon oxychlorides or thionyl chloride in the presence of a catalyst adduct of an N,N-disubstituted formamide of general formula (I) and carbon oxychloride or thionyl chloride. In the formula, R 1  and R 2 , independently from one another, mean C 1 - to C 4 -alkyl or R 1  and R 2  together mean a C 4 - or C 5 -alkylene chain. According to the inventive method, hydrogen chloride is added during and/or after the conversion.

The present invention relates to a process for the preparation ofcarbonyl chlorides by reacting the corresponding carboxylic acids withphosgene or thionyl chloride in the presence of a catalyst adduct withthe simultaneous and/or subsequent introduction of hydrogen chloride,which leads to carbonyl chlorides which have a low color number.

Carbonyl chlorides are important intermediates in the synthesis of alarge number of chemical products, in particular pharmaceuticals,cosmetics, surfactants and paper auxiliaries. They can be prepared byreacting carboxylic acids with chlorinating agents, such as PCl₃, POCl₃,SOCl₂, SO₂Cl₂ or COCl₂. Of industrial importance are, in particular, thereactions with thionyl chloride, phosphorus trichloride and phosgene.

As a rule, in the synthesis via phosphorus trichloride, one reactant(carboxylic acid or phosphorus trichloride) is initially introduced, andthe other reactant (phosphorus trichloride or carboxylic acid) is slowlyadded. Where appropriate, the synthesis is carried out in a solutiondiluted with a reaction-inert solvent (e.g. toluene). After removal ofthe phosphorous acid formed, the carbonyl chloride is as a rule purifiedby distillation. The addition of a catalyst is not required.

EP-A-0 296 404 describes the purification of crude carbonyl chlorideswhich originate from the chlorination using phosphorus trichloride, inwhich the reaction products are treated with carboxamide hydrohalides.The crude carbonyl chloride solutions from the phosphorus trichlorideroute differ in composition from those obtainable by the phosgene orthionyl chloride route. For example, the latter have:

(i) a considerably higher content of undesired minor components.

(ii) a varying composition of the minor components, which is influencedby the choice of chlorinating agent.

(iii) supplementary to the varying composition of the minor components,also the presence of degradation and/or secondary products from thecatalyst adducts used.

The use of phosgene or thionyl chloride instead of phosphorustrichloride generally leads to a higher conversion and betterselectivity. Both chlorinating agents additionally have the advantageover phosphorus trichloride that only gaseous byproducts are formed,which either escape in the form of gas during the synthesis or can becompletely expelled by stripping with an inert gas when the reaction iscomplete. Furthermore, phosgene, in particular, is a very good valuechlorinating agent.

Thionyl chloride and, in particular, phosgene are less reactive aschlorinating agents compared with phosphorus trichloride. Thepreparation of carbonyl chlorides by reacting carboxylic acids withthionyl chloride is therefore preferably carried out in the presence ofa catalyst to increase the reaction rate. In the preparation by reactionwith phosgene, a catalyst is always used. Catalyst precursors which aresuitable for both chlorinating agents are N,N-disubstituted formamidesand hydrochlorides thereof, and also pyridine or urea. Overviewsrelating to the chlorination by means of thionyl chloride are given inM. F. Ansell in S. Patai, “The Chemistry of Acyl Halides”, John Wileyand Sons, New York 1972, 35-69 and H. H. Bosshard et al., Helv. Chem.Acta 62 (1959) 1653-1658 and S. S. Pizey, Synthetic Reagents, Vol. 1,John Wiley and Sons, New York 1974, ISBN 853120056, 321-557, inparticular 333-335. Both by the phosgene route and also by the thionylchloride route preference is given to using N,N-disubstitutedformamides. These react with said chlorinating agents to give theVilsmeier salts.

The Vilsmeier salt, the actual reactive chlorinating reagent, reactswith the carboxylic acid or the carboxylic anhydride to give the acidchloride. In the process, formamide-hydrochloride is reformed, which canin turn react with phosgene or thionyl chloride to give the Vilsmeiersalt and passes through further catalyst circuits. The N,N-disubstitutedformamide-hydrochlorides or vilsmeier salts thereof are not, however,very thermally stable, meaning that it is possible for secondaryreactions to take place above 80 to 90° C.

The preferred use of N,N-disubstituted formamides as catalyst precursorfor the phosgenation of carboxylic acids also emerges from EP-A-0 367050, EP-A-0 452 806, DE-A-4 337 785, EP-A-0 475 137 and EP-A-0 635 473.

As regards the color number, in the chlorination of carboxylic acidsusing phosgene or thionyl chloride, the use of catalysts has an adverseeffect. Although these catalysts are separated off by phase separationfollowing the chlorination, they can, however, remain in the product insmall amounts and lead either themselves or as degradation or secondaryproducts to yellow colorations of the carbonyl chlorides. For thisreason, the carbonyl chlorides prepared via phosgene or thionyl chlorideare generally purified by distillation to give largely colorlessproducts. Such a distillation is not only an energy- and time-consumingoperation, but also harbors a number of further disadvantages. Manylonger-chain carbonyl chlorides cannot be distilled without partialdecomposition. Furthermore, it is known that the distilled products canbecome contaminated as a result of decomposition of the catalyst stillpresent in the distillation bottoms. Relatively large amounts ofaccumulated catalyst residue also represent a safety risk during thedistillation since at elevated temperature there is the risk ofspontaneous decomposition.

A further method of purifying the crude carbonyl chlorides is thetreatment with activated carbons. However, these absorptive purificationsteps are industrially complex and, moreover, are not always successful.In addition, a contaminated solid forms, which has to be subsequentlydisposed of in the correct manner.

It is an object of the invention to develop a process for thepreparation of carbonyl chlorides by reacting the correspondingcarboxylic acids with phosgene or thionyl chloride which no longer hasthe known disadvantages and leads to carbonyl chlorides which have a lowcolor number.

We have found that this object is achieved by the development of aprocess for the preparation of carbonyl chlorides by reacting carboxylicacids with phosgene or thionyl chloride in the presence of a catalystadduct of an N,N-disubstituted formamide of the formula (I)

in which R¹ and R² independently of one another are C₁- to C₄-alkyl, orR¹and R² together are a C₄- or C₅-alkylene chain, and phosgene orthionyl chloride, which comprises introducing hydrogen chloride duringand/or after the reaction.

By the process according to the invention, it is possible to preparecarbonyl chlorides by reacting the corresponding carboxylic acids withphosgene or thionyl chloride in high yield and with low color number. Alow color number here means a color number which is at most 50% of theAPHA color number, or in the case of unsaturated carbonyl chlorides, atmost 75% of the iodine color number which is achieved using the processof the prior art, i.e. without the inventive measure. The determinationsof the APHA color number and of the iodine color number are described inthe standard DIN EN 1557 (March 1997).

The inventive introduction of the hydrogen chloride can be carried outin a variety of ways. For example, the hydrogen chloride can beintroduced, with regard to the introduction of the chlorinating agentphosgene or thionyl chloride, exclusively during its addition, duringand after its addition or exclusively after its addition. Preference isgiven to metering in the hydrogen chloride at the same time as thechlorinating agent is added. In the three variants mentioned, thehydrogen chloride can be introduced continuously, i.e. withoutinterruption, or with one or more interruptions, to a pulse-like meteredaddition. In addition, the rate of addition of the hydrogen chloridewithin an addition interval can remain constant or can decrease orincrease. Within the meaning of a constant working of the reaction, itis advantageous to introduce the hydrogen chloride continuously, wherean interruption, for example in the sense of a subsequent increase inthe hydrogen chloride concentration, can still be entirely advantageous.

For the process according to the invention, it is immaterial whether thehydrogen chloride is added at one site together with the chlorinatingagent or at another site, spatially separate from the chlorinatingagent. What is essential, however, is that the reaction solution isthoroughly mixed during the reaction with the chlorinating agent, andalso during the introduction of the hydrogen chloride, and the presenceof the catalyst phase during the introduction of hydrogen chloride. Thecatalyst phase is advantageously separated off only after all of thehydrogen chloride has been added.

In the preparation of the carbonyl chlorides according to the invention,the catalyst used is a catalyst adduct formed from the reaction ofphosgene or thionyl chloride with an N,N-disubstituted formamide. Thelatter, which is also referred to as a catalyst precursor, is defined bythe formula (I)

in which R¹ and R² independently of one another are a C₁- to C₄-alkyl,specifically methyl, ethyl, propyl, 1-methylethyl, butyl,1-methylpropyl, 2-methylpropyl and 1,1-dimethylethyl, or together are aC₄- or C₅-alkylene chain, specifically CH₂CH₂CH₂CH₂ or CH₂CH₂CH₂CH₂CH₂.

It is important that the mutual solubility of the carbonyl chlorides andof the hydrochlorides of the N,N-disubstituted formamides (I) formed bythe introduction of hydrogen chloride is low and that two isolatablephases form.

Preference is given to using N,N-dimethylformamide.

The formation of the catalyst adduct can either be carried out in theapparatus where the chlorination is carried out, or else upstream inanother apparatus. In the last-mentioned case, a certain amount of theN,N-disubstituted formamide is introduced into a separate apparatus andsaturated with hydrogen chloride, and the desired amount of phosgene orthionyl chloride is introduced. The mixture can then be transferred tothe actual reaction apparatus. In the first-mentioned case, theprocedure described is carried out directly in the reaction apparatus.Preference is given to bringing the carboxylic acid into contact withthe N,N-disubstituted formamide (I) and subsequently simultaneouslyintroducing the chlorinating agent and the hydrogen chloride. If theprocess is operated with catalyst recycle, the carboxylic acid isbrought into contact with the recycled catalyst and optionally freshN,N-disubstituted formamide (I), and then the chlorinating agent andhydrogen chloride are introduced as described above.

The amount of N,N-disubstituted formamide (I) to be used is dependent onthe type of chlorinating agent. If phosgene is used, a molar amount ofN,N-disubstituted formamide (I) of from 0.05 to 2.0 is advantageouslyused, preferably from 0.1 to 1.0 and particularly preferably from 0.1 to0.6, based on the molar amount of carboxylic acid used. If thionylchloride is used, the corresponding advantageous range is between 0.001and 0.05 and preferably between 0.001 and 0.01.

The reaction between the carboxylic acid and phosgene or thionylchloride is generally carried out at temperatures from 0 to 100° C.,preferably from 20 to 80° C., particularly preferably from 20 to 60° C.The reaction is generally carried out at a pressure of from 0.5 to 2.0bar abs, preferably from 0.8 to 1.2 bar abs, particularly preferably atatmospheric pressure. Suitable reaction apparatuses which may bementioned are the apparatuses known to the person skilled in the art forreactions in the liquid/liquid and gas/liquid phase, such as, forexample, stirred-tank reactors or batteries of stirred-tank reactorswith appropriate gas inlet and gas distribution technology.

The molar amount of phosgene or thionyl chloride added overall duringthe reaction to give the carbonyl chloride is from 1.0 to 2.0, based onthe molar amount of the carboxylic acid used. Preference is given to amolar amount of from 1.0 to 1.3, based on the molar amount of thecarboxylic acid used.

The molar amount of hydrogen chloride to be introduced overall duringthe process according to the invention is dependent on the molar amountof carboxylic acid used and is advantageously in the range between 0.2and 2.0, based on the molar amount of carboxylic acid used. Preferenceis given to a molar amount of from 0.5 to 1.5, based on the molar amountof carboxylic acid used. As already explained above, in the processaccording to the invention, hydrogen chloride can be introduced duringand/or after the reaction of the carboxylic acids with phosgene orthionyl chloride. The specified molar amount of hydrogen chloridecorresponds to the cumulative molar amount over the entire process. Inthe case of a continuous procedure, the relative molar amounts given areto refer to the time unit where, in this case, both the molar amount ofthe freshly introduced N,N-disubstituted formamide (I) and also that ofthe recycled catalyst are to be used.

Following the reaction with the chlorinating agent, the reaction mixturecan be thoroughly mixed for a further period, where, depending on theembodiment, further hydrogen chloride can also be introduced. Thesubsequent, thorough mixing is generally carried out for one hour atmost, but, depending on the reaction system and desired product purity,can also be dispensed with. In addition, it is also possible, afteraddition of the chlorinating agent is complete, to add yet furtherN,N-disubstituted formamide, preferably with the further introduction ofhydrogen chloride or as a hydrochloride, and mix thoroughly. This can,for example, be added following the chlorination using thionyl chloridein order to increase the amount of extractant.

An essential feature for achieving a low color number of the carbonylchlorides prepared is the composition of the catalyst-containing phasefollowing the reaction. The lower the content of catalyst adduct, thelower, too, the achievable color number of the carbonyl chlorides. Themolar content of the catalyst adduct, based on the total molar amount ofN,N-disubstituted formamide (I) plus catalyst adduct, is advantageouslyless than 0.3 according to the process according to the invention.Preference is given to a relative content of less than 0.1, particularlypreferably of less than 0.05. The relative content can be adjusted viathe added amount of chlorinating agent and of hydrogen chloride.

The carbonyl chlorides and the catalyst-containing phase areadvantageously isolated by phase separation. This can be carried outeither in the reaction apparatus used above, provided it is suitable forthis purpose, or else in a separate apparatus. Suitable apparatuses are,for example, stirred-tank reactors, batteries of stirred-tank reactorsor phase-separating vessels, such as “mixer settlers”. In general, bothphases have separated within 2 hours. For isolation, it is also possibleto use suitable filters, such as, for example, coalescence filters ofknown construction.

The carbonyl chlorides prepared in this way have a considerably lowercolor number than carbonyl chlorides prepared in accordance with theprior art without the measure according to the invention, and can thenas a rule be used directly for further synthesis stages. If required,they can, however, also be subjected to still further treatmentprocedures. Examples which may be mentioned are the treatment with ahydrochloride of an N,N-disubstituted formamide, distillation oradsorptive purification.

According to an advantageous embodiment of the process, theseparated-off catalyst-containing phase, comprising N,N-disubstitutedformamide (I) and catalyst adduct, can be reused as catalyst precursorin the further synthesis. For this, the catalyst-containing phase isreturned to the synthesis stage, as already described. It isadvantageous to bleed some of the catalyst-containing phase from thesystem in order to avoid a buildup of undesired secondary components.

The process according to the invention can either be carried outbatchwise or continuously.

(a) batchwise preparation:

In the batchwise preparation, the reaction mixture, consisting of thecarboxylic acid and the N,N-disubstituted formamide (I) or the catalystadduct, prepared from phosgene or thionyl chloride and theN,N-disubstituted formamide (I), is introduced into a reactionapparatus, for example a stirred-tank reactor. Then, the desired amountof liquid or gaseous phosgene or thionyl chloride, and, in parallelthereto, the desired amount of hydrogen chloride is added over a certainperiod of time. The time requirement for the addition of thechlorinating agent depends on the reaction rate and can generally belimited to a few hours. Depending on the embodiment, in one variant, theintroduction of hydrogen chloride ends with the completion of theaddition of the chlorinating agent or, in another variant, is maintainedbeyond that. When the addition of hydrogen chloride is complete, thereaction solution is generally left to stand for 1 to 2 hours, and thetwo phases are separated from one another. As a rule, thecarbonyl-chloride-containing phase is the upper one, and thecatalyst-containing phase is the lower one.

It may be expressly pointed out that, in a third variant, theintroduction of hydrogen chloride according to the invention can also bestarted only after the introduction of the chlorinating agent iscomplete. In this case, the reaction with the chlorinating agent wouldtake place without the introduction of hydrogen chloride.

(b) continuous preparation:

Reaction apparatuses suitable for the continuous procedure are, forexample, stirred-tank reactors, batteries of stirred-tank reactors orreaction columns operated countercurrently. If a stirred-tank reactor isused, the carboxylic acid and the N,N-disubstituted formamide (I) or thecatalyst adduct, prepared from phosgene or thionyl chloride and theN,N-disubstituted formamide (I), are introduced, and liquid or gaseousphosgene or thionyl chloride and, in parallel thereto, the desiredamount of hydrogen chloride are added. After an amount of chlorinatingagent which is approximately equivalent to the carboxylic acid has beenintroduced, the simultaneous introduction of carboxylic acid andN,N-disubstituted formamide (I) and catalyst adduct, and an amount ofphosgene or thionyl chloride which is essentially equimolar to the addedcarboxylic acid, is started. In addition, the desired amount of hydrogenchloride is introduced continuously. An amount of the reaction volumecorresponding to the reactants introduced is removed from the reactionapparatus, for example by maintaining the level, and passed to aseparating vessel. In the separating vessel, the carbonyl chloride canbe continuously removed as the upper phase, and the catalyst-containing,lower phase can be continuously returned to the reactor. In implementingthe reaction, it is to be ensured that the chlorinating agent entrainedby the reaction exit gases is replaced by introducing additionalchlorinating agent.

It may expressly be pointed out that, in a further variant of theprocess according to the invention, further hydrogen chloride can beadded even after discharge from the reaction apparatus. This can becarried out, for example, in a further apparatus, for example astirred-tank reactor, situated between the reaction apparatus and theseparating vessel. In addition, it is also possible to only introducethe hydrogen chloride subsequently. In this case, the reaction with thechlorinating agent takes place without the introduction of hydrogenchloride.

Preferably, the carbonyl chlorides are prepared by the process accordingto the invention by reacting the corresponding carboxylic acids withphosgene as chlorinating agent.

Carbonyl chlorides which can be prepared by the process according to theinvention are, for example, those of the formula (II)

in which R stands for the following radicals:

C₁- to C₃₀-alkyl or their aryl- or cycloalkyl-substituted components:

saturated, straight-chain or branched hydrocarbon radical having from 1to 30 carbon atoms, preferably methyl, ethyl, propyl, 1-methylethyl,butyl, 1-methylpropyl, 2-methylpropyl, 1,1-dimethylethyl, pentyl,1-ethylpropyl, hexyl, heptyl, 1-ethylpentyl, octyl,2,4,4-trimethylpentyl, nonyl, 1,1-dimethylheptyl, decyl, undecyl,dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl,octadecyl, nonadecyl, icosyl, henicosyl, docosyl, tricosyl, tetracosyl,pentacosyl, hexacosyl, heptacosyl, octacosyl, nonacosyl, triacontyl,phenylmethyl, diphenylmethyl, triphenylmethyl, 2-phenylethyl,3-phenylpropyl, cyclopentylmethyl, 2-cyclopentylethyl,3-cyclopentylpropyl, cyclohexylmethyl, 2-cyclohexylethyl,3-cyclohexylpropyl;

C₃- to C₁₂-cycloalkyl or their aryl- or cycloalkyl-substitutedcomponents:

monocyclic, saturated hydrocarbon radical having from 3 to 12 ringcarbon atoms, preferably cyclopentyl, cyclohexyl;

C₂- to C₃₀-alkenyl or their aryl- or cycloalkyl-substituted components:

unsaturated, straight-chain or branched hydrocarbon radical having from1 to 30 carbon atoms and 1 to 5 double bonds at any position, preferably2-propenyl, 3-butenyl, cis-2-butenyl, trans-2-butenyl,cis-8-heptadecenyl, trans-8-heptadecenyl, cis,cis-8,11-heptadecadienyl,cis,cis,cis-8,11,14-heptadecatrienyl;

C₃- to C₁₂-cycloalkenyl or their aryl- or cycloalkyl-substitutedcomponents:

monocyclic, unsaturated hydrocarbon radical having from 3 to 12 ringcarbon atoms and 1 to 3 double bonds at any position, preferably3-cyclopentenyl, 2-cyclohexenyl, 3-cyclohexenyl, 2,5-cyclohexadienyl;

C₂- to C₃₀-alkynyl or their aryl- or cycloalkyl-substituted components:

unsaturated, straight-chain or branched hydrocarbon radical having from1 to 30 carbon atoms and 1 to 3 triple bonds at any position, preferably3-butynyl, 4-pentynyl;

C₄- to C₃₀-alkenynyl or their aryl- or cycloalkyl-substitutedcomponents:

unsaturated, straight-chain or branched hydrocarbon radical having from1 to 30 carbon atoms, 1 to 3 triple bonds and 1 to 3 double bonds at anyposition.

Using the process according to the invention, it is also possible toprepare mixtures of said carbonyl chlorides. Non-limiting examples whichmay be mentioned are mixtures comprising C₈- to C₁₈-carbonyl chlorides,which are traded under the trivial names “carboxylic acid chloride”,“tallow fatty acid chloride”, “coconut fatty acid chloride” and “oleicacid chloride”.

Particular preference is given to preparing carbonyl chlorides of theformula (III) by the process according to the invention, in which Rstands for the following radicals:

C₁- to C₃₀-alkyl or their aryl- or cycloalkyl-substituted components:

saturated, straight-chain or branched hydrocarbon radical having from 1to 30 carbon atoms, preferably methyl, ethyl, propyl, 1-methylethyl,butyl, 1-methylpropyl, 2-methylpropyl, 1,1-dimethylethyl, pentyl,1-ethylpropyl, hexyl, heptyl, 1-ethylpentyl, octyl,2,4,4-trimethylpentyl, nonyl, 1,l-dimethylheptyl, decyl, undecyl,dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl,octadecyl, nonadecyl, icosyl, henicosyl, docosyl, tricosyl, tetracosyl,pentacosyl, hexacosyl, heptacosy, octacosyl, nonacosyl, triacontyl,phenylmethyl, diphenylmethyl, triphenylmethyl, 2-phenylethyl,3-phenylpropyl, cyclopentylmethyl, 2-cyclopentylethyl,3-cyclopentylpropyl, cyclohexylmethyl, 2-cyclohexylethyl,3-cyclohexylpropyl;

C₂- to C₃₀-alkenyl or their aryl- or cycloalkyl-substituted components:

unsaturated, straight-chain or branched hydrocarbon radical having from1 to 30 carbon atoms and 1 to 5 double bonds at any position, preferably2-propenyl, 3-butenyl, cis-2-butenyl, trans-2-butenyl,cis-8-heptadecenyl, trans-8-heptadecenyl, cis,cis-8,11-heptadecadienyl,cis,cis,cis-8, 11, 14-heptadecatrienyl;

and mixtures thereof.

Very particularly preferably, the process according to the invention isused to prepare acetyl chloride (R=methyl), propionyl chloride(R=ethyl), butyryl chloride (R=propyl), valeryl chloride (R=butyl),isovaleryl chloride (R=2-methylpropyl), pivaloyl chloride(R=1,1-dimethylethyl), caproyl chloride (R=pentyl), 2-ethylbutyrylchloride (R=1-ethylpropyl), enanthyl chloride (R=hexyl), capryloylchloride (R=heptyl, 2-ethylhexanoyl chloride (R=1-ethylpentyl),pelargonoyl chloride (R=octyl), isononanoyl chloride(R=2,4,4-trimethylpentyl), capryl chloride (R=nonyl), neodecanoylchloride (R=1,1-dimethylheptyl), lauroyl chloride (R=undecyl), myristoylchloride (R=tridecyl), palmitoyl chloride (R=pentadecyl), stearoylchloride (R=heptadecyl), oleoyl chloride (R=cis-8-heptadecenyl),linoleoyl chloride (R=cis, cis-8,11-heptadecadienyl), linolenoylchloride (R=cis,cis,cis-8,11,14-heptadecatrienyl), arachidoyl chloride(R=nonadecyl) and behenoyl chloride (R=henicosyl)

and mixtures thereof.

The carboxylic acids to be used for the process according to theinvention arise from the above-described definitions for R. It may bepointed out that mixtures of the various carboxylic acids can also bechlorinated using the process according to the invention.

In a general variant for the batchwise preparation of the carbonylchlorides by chlorination by means of thionyl chloride, the total amountof the corresponding carboxylic acid is initially introduced into astirred tank reactor, and the required amount of N,N-disubstitutedformamide (I) is metered in with stirring. The reaction system is thenbrought to the desired temperature, and liquid thionyl chloride iscontinuously metered in at atmospheric pressure with further vigorousstirring. The gaseous products sulfur dioxide and hydrogen chlorideformed are drawn off. With regard to the amount of thionyl chlorideintroduced, it is to be ensured that only a low concentration ofcatalyst adduct is still present following the reaction. When thethionyl chloride addition is complete, N,N-disubstituted formamide isagain added with further stirring. Gaseous hydrogen chloride is thenintroduced where, in the determination of the amount introduced, thetotal amount of N,N-disubstituted formamide added, i.e. before and afterchlorination, is decisive. For the addition rate of the hydrogenchloride introduction, a good uptake in the reaction solution is to beensured. If necessary, the rate of addition must be lowered. Also afterthe hydrogen chloride addition is complete, the reaction solution can befurther stirred. The mixing is then stopped so that the two phases canseparate. The lower, catalyst-containing phase is separated off and canbe reused in further syntheses. The carbonyl chloride phase which isleft is freed from residual hydrogen chloride and sulfur dioxide bypassing nitrogen through it, and can then be used for further synthesisstages, usually without additional purification steps.

In a general variant for the batchwise preparation of the carbonylchlorides by chlorination by means of phosgene, the total amount of thecorresponding carboxylic acid is introduced into a stirred tank reactor,and the required amount of N,N-disubstituted formamide (I) is metered inwith stirring. The reaction system is then brought to the desiredtemperature and, at atmospheric pressure and with further vigorousstirring, gaseous or liquid phosgene and gaseous hydrogen chloride arecontinuously introduced until the equimolar amount with regard to thecarboxylic acid used and additionally a slight excess of phosgene havebeen introduced into the reaction mixture. The gaseous products carbondioxide and hydrogen chloride formed are drawn off. With regard to theamount of phosgene added, it must be ensured that only a lowconcentration of catalyst adduct is still present following thereaction. Depending on the variant, it is possible to stop theintroduction of the hydrogen chloride with the end of the phosgeneaddition, or else to continue beyond the end of the phosgene addition.The total amount of hydrogen chloride introduced should, however, basedon the amount of N,N-disubstituted formamide (I), advantageously bewithin the range given. After the addition of the hydrogen chloride iscomplete, the reaction solution is stirred for a further 1 to 2 hours.The mixing is then stopped so that the two phases can separate. Thelower, catalyst-containing phase is separated off and can be reused infurther syntheses. The carbonyl chloride phase which is left is freedfrom residual hydrogen chloride and carbon dioxide by passing nitrogenthrough it, and can then be used for further synthesis stages, usuallywithout additional purification steps.

In a general variant for the continuous preparation of the carbonylchlorides by chlorination by means of phosgene, the carboxylic acid,recycled catalyst adduct, optionally fresh N,N-disubstituted formamide(I), gaseous or liquid phosgene and gaseous hydrogen chloride arecontinuously introduced into a stirred tank reactor with vigorousstirring at the desired temperature under atmospheric pressure. The rateof addition of the phosgene is dependent on that of the carboxylic acid,and the rate of addition of the hydrogen chloride is dependent on thatof the catalyst adduct or of the N,N-disubstituted formamide (I).Particularly in the case of the introduction of.the phosgene, it must beensured that only a low concentration of catalyst adduct is stillpresent in the drawn-off solution. An amount corresponding to theintroduced amount is continuously drawn off from the stirred tankreactor and passed to a separating vessel. From this, thecatalyst-containing phase, which is generally the lower phase, iscontinuously drawn off and returned to the stirred tank reactor. Thecarbonyl chloride phase which is left is removed from the separatingvessel and, in a further vessel, freed from residual hydrogen chlorideand carbon dioxide by passing nitrogen through it. It can then be usedfor further synthesis stages, usually without additional purificationsteps.

A further general variant for the continuous preparation of the carbonylchlorides by chlorination by means of phosgene differs from that justdescribed in that the phosgenation is carried out in a stirred tankreactor or a battery of stirred tank reactors consisting of 2 to 3stirred tank reactors, without the introduction of hydrogen chloride.The reaction solution which is continuously drawn off is passed to afurther stirred tank reactor situated between the stirred tank reactoror the battery of stirred tank reactors and the separating vessel. Here,the continuous treatment with the hydrogen chloride takes place. Theextracted reaction solution is then passed to the separating vessel andfurther treated as described above.

An essential feature of the preparation of the carbonyl chlorides havinga low color number according to the invention and as described above isthe surprising effect that precisely the color-imparting components areconsiderably more soluble in the hydrogen-chloride-containing phase ofthe N,N-disubstituted formamide (I) than in thecarbonyl-chloride-containing phase.

The process according to the invention leads, mainly by virtue of themeasure of introducing hydrogen chloride, which can be readilyintegrated into the synthesis process, to carbonyl chlorides with a lowcolor number, meaning that they can be usually used for subsequentreactions without distillation, separate extraction or adsorptivetreatment. The process according to the invention can be carried outvery effectively and economically. By avoiding the distillation which iscustomary according to the prior art, both investment and energy costsare saved, and also as a rule a higher yield of purified carbonylchloride is achieved. For distillation-sensitive carbonyl chlorides, theprocess according to the invention opens up the possibility of aneconomical synthesis on an industrial scale.

EXAMPLES Comparative Example 1

Preparation of Lauroyl Chloride

82.2 g (1.13 mol) of N,N-dimethylformamide were added to 4.5 mol oflauric acid in a stirred apparatus. The reaction solution was brought toa temperature of from 40 to 50° C. with stirring, and a total of 5.06mol of gaseous phosgene were introduced under atmospheric pressure.After the addition of phosgene was complete, the two phases wereseparated from one another. The catalyst phase comprised a molarproportion of the catalyst adduct, based on the molar amount ofN,N-dimethylformamide plus catalyst adduct, of 0.50. The carbonylchloride phase comprised 99.1 area % of lauroyl chloride and 0.15 area %of lauric acid. The color number was 268 APHA.

As a result of a relatively high molar ratio between the phosgeneintroduced and the lauric acid used, a high conversion to lauroylchloride was achieved. The carbonyl-chloride-containing phase, however,has an unsatisfactory, high color number.

Comparative Example 2

Preparation of Pelargonoyl Chloride (Nonanoyl Chloride)

100.5 g (1.38 mol) of N,N,-dimethylformamide were added to 2.75 mol ofpelargonic acid in a stirred apparatus. The reaction solution wasbrought to a temperature of from 20 to 30° C. with stirring, and a totalof 2.78 mol of gaseous phosgene were introduced under atmosphericpressure. After the addition of phosgene was complete, the two phaseswere separated from one another. The catalyst phase comprised a molarproportion of the catalyst adduct, based on the molar amount ofN,N-dimethylformamide plus catalyst adduct, of <0.05. The carbonylchloride phase comprised 97.1 area % of pelargonoyl. chloride and 1.9area % of pelargonic anhydride. The color number was 16 APHA.

As a result of a very low, virtually stoichiometric molar ratio betweenthe phosgene introduced and the pelargonic acid used, only anunsatisfactorily low content of pelargonoyl chloride was achieved in thecrude product, with too high a content of pelargonic anhydride. However,the carbonyl-chloride-containing phase exhibits a very low color number.

Example 3

Preparation of Pelargonoyl Chloride (Nonanoyl Chloride)

100.5 g (1.38 mol) of N,N-dimethylformamide were added to 2.75 mol ofpelargonic acid in a stirred apparatus. The reaction solution wasbrought to a temperature of from 20 to 30° C. with stirring, and a totalof 2.78 mol of gaseous phosgene and simultaneously 1.92 mol of gaseoushydrogen chloride were introduced under atmospheric pressure. When theaddition of phosgene and hydrogen chloride was complete, the two phaseswere separated from one another. The catalyst phase comprised a molarproportion of the catalyst adduct, based on the molar amount ofN,N-dimethylformamide plus catalyst adduct, of 1%. The carbonyl chloridephase comprised 98.9% by weight of pelargonoyl chloride and 0.04% byweight of pelargonic anhydride. The color number was 18 APHA.

Only as a result of the simultaneous introduction of hydrogen chlorideaccording to the invention was it possible to obtain a high conversionto pelargonoyl chloride having a very low color number.

Comparative Example 4

Preparation of Coconut Fatty Acid Chloride

36.6 g (0.5 mol) of N,N-dimethylformamide were added to 2.0 mol ofcoconut fatty acid (trade name HK 8-18, Henkel), which consistsessentially of lauric acid and myristic acid, in a stirred apparatus.The reaction solution was brought to a temperature of 30° C. withstirring, and a total of 2.38 mol of gaseous phosgene were introducedunder atmospheric pressure. After the addition of phosgene was complete,the two phases were separated from one another. The catalyst phasecomprised a molar proportion of the catalyst adduct, based on the molaramount of N,N-dimethylformamide plus catalyst adduct, of 0.50. Thecarbonyl chloride phase comprised 99.6% by weight of coconut fatty acidchloride and 0.35% by weight of coconut fatty acid. The color number was399 APHA.

As a result of a relatively high molar ratio between the phosgeneintroduced and the coconut fatty acid used, a high conversion to coconutfatty acid chloride was achieved. However, thecarbonyl-chloride-containing phase exhibits an unsatisfactory, highcolor number.

Example 5

Preparation of Coconut Fatty Acid Chloride

73.1 g (1.0 mol) of N,N-dimethylformamide were added to 2.01 mol ofcoconut fatty acid (trade name HK 8-18, Henkel) in a stirred apparatus.The reaction solution was brought to a temperature of 30° C. withstirring, and a total of 2.1 mol of gaseous phosgene and simultaneously1.04mol of gaseous hydrogen chloride were introduced under atmosphericpressure. When the addition of phosgene and hydrogen chloride werecomplete, the two phases were separated from one another. The catalystphase comprised a molar proportion of the catalyst adduct, based on themolar amount of N,N-dimethylformamide plus catalyst adduct, of <0.10.The carbonyl chloride phase comprised 99.5% by weight of coconut fattyacid chloride and 0.5% by weight of coconut fatty acid. The color numberwas 44 APHA.

Only as a result of the simultaneous introduction of hydrogen chlorideaccording to the invention was it possible to obtain a high conversionto coconut fatty acid chloride with a very low color number.

The examples show that, irrespective of the type of carboxylic acid, bysimultaneously introducing hydrogen chloride gas in the reaction withthe chlorinating agent, a high conversion to the desired carbonylchloride with very low color numbers is achieved. The carbonyl chloridesobtained in the examples according to the invention can be used insubsequent syntheses without further purification steps.

We claim:
 1. A process for the preparation of carbonyl chlorides byreacting carboxylic acids with phosgene or thionyl chloride in thepresence of a catalyst adduct of an N,N-disubstituted formamide of theformula (I) in which R¹ and R² independently of one another are C₁- toC₄-alkyl or R¹ and R²

together are a C₄- or C₅-alkylene chain and phosgene or thionylchloride, which comprises introducing gaseous hydrogen chloride duringthe reaction.
 2. A process as claimed in claim 1, wherein, overall, amolar amount of gaseous hydrogen chloride of 0.2 to 2.0, based on themolar amount of carboxylic acid employed, is used.
 3. A process asclaimed in claim 1, wherein, in the reaction with phosgene, a molaramount of N,N-disubstituted formamide (I) of 0.05 to 2.0, based on themolar amount of carboxylic acid employed, is used.
 4. A process asclaimed in claim 1, wherein, in the reaction with thionyl chloride, amolar amount of N,N-disubstituted formamide (I) of 0.001 to 0.05, basedon the molar amount of carboxylic acid employed, is used.
 5. A processas claimed in claim 1, wherein, during the reaction, a molar amount ofphosgene or thionyl chloride of 1.0 to 2.0, based on the molar amount ofcarboxylic acid, is used.
 6. A process as claimed in claim 1, whereinthe molar proportion of the catalyst adduct of the N,N-disubstitutedformamide (I) and phosgene or thionyl chloride, based on the molaramount of N,N-disubstituted formamide (I) plus catalyst adduct, is lessthan 0.3 after the reaction.
 7. A process as claimed in claim 1, whereinthe molar proportion of the catalyst adduct of the N,N-disubstitutedformamide (I) and phosgene or thionyl chloride, based on the molaramount of N,N-disubstituted formamide (I) plus catalyst adduct, is lessthan 0.1 after the reaction.
 8. A process as claimed in claim 1, whereinthe carbonyl chloride is isolated from the reaction mixture followingthe reaction by phase separation.
 9. A process as claimed in claim 1,wherein the N,N-disubstituted formamide (I) used isN,N-dimethylformamide.
 10. A process as claimed in claim 1, wherein,following the reaction, the N,N-disubstituted formamide (I), itshydrochloride and catalyst adduct are separated off and reused ascatalyst precursor in the carbonyl chloride synthesis.
 11. A process asclaimed in claim 1, wherein the carboxylic acids are reacted withphosgene.
 12. A process as claimed in claim 1, wherein the carbonylchlorides prepared are acetyl chloride, propionyl chloride, butyrylchloride, valeryl chloride, isovaleryl chloride, pivaloyl chloride,caproyl chloride, 2-ethylbutyryl chloride, enanthyl chloride, capryloylchloride, 2-ethylhexanoyl chloride, pelargonoly chloride, isononanoylchloride, capryl chloride, neodecanoyl chloride, lauroyl chloride,myristoyl chloride, palmitoyl chloride, stearoyl chloride, oleoylchloride, linoleoyl chloride, linolenoyl chloride, arachidoyl chlorideand behenoyl chloride, and mixtures thereof.